This invention relates to the art of video display of images and, more particularly, to improvements in modifying or enhancing the visual presentation of images representing data characters and the like.
Whereas the invention is described herein in conjunction with a video display system incorporating a host computer and individual terminals, it is to be appreciated that the invention is not limited thereto but may also be employed in a stand-alone video display terminal or other apparatus for displaying graphical images wherein it is desirable to provide video modifications or enhancements thereto.
Video display systems which employ terminals having means for displaying data characters as well as for modifying the video characteristics of the displayed characters are known in the art. Examples of such display systems include the U.S. Pat. Nos. to R. C. Williams 3,895,374, 3,895,375 and 3,896,428. The display terminal described in those patents has the ability to display a data character as a dot pattern within a dot matrix. A horizontal T.V. scan is employed so that as the beam sweeps across the face of the display screen it forms horizontal dot slices of each character in a line of characters. This is repeated for several scans until a line of data characters is presented. In addition, the terminals described in these patents also provide video enhancements, which are referred to in the patents as display attributes. These refer to the changing of the normal display of a data character from being merely a dot pattern to a variation in its video appearance, such as: inverted in appearance, intensified or underlined. In those terminals, the data characters are encoded in binary form and preceded in the data stream by a binary encoded attribute character which includes attribute bits specifying the display attributes which the data characters are to have when displayed.
A notable disadvantage of providing attributes as discussed above is that the terminal must employ logic circuitry to determine, in conjunction with the forming of the image of a data character, whether or not the attribute for that data character is to be changed. This is because each attribute character is followed by one or more data characters until the particular attribute is to be changed and then a new attribute character appears in the data stream. Consequently, logic circuitry is required to continuously monitor the data stream to determine if the attribute has been changed. Since the attribute character is not to be displayed itself there results a non-continuous data stream of data characters intermixed with attribute characters at irregular occurring intervals. Consequently, irregularly occurring time spaces result when no data characters are being displayed (because an attribute character is being decoded) and special logic circuitry must be employed if the data characters are to be generated for display at regularly occurring times.